Linear transimpedance amplifier with wide dynamic range for high rate applications

ABSTRACT

Various amplifier configurations having increased bandwidth, linearity, dynamic range, and less distortion are shown and disclosed. To increase bandwidth in a transimpedance amplifier, a replica circuit is created to replicate a degeneration resistance, or the resistance or value that relates to a feedback resistance. From the replica circuit, the replicated values are mirrored and processed to control a FET switch which modifies a degeneration resistance. The FET switch control signal is related to the feedback resistance and modifies the degeneration resistance to thereby maintain the product of the feedback resistance and the degeneration resistance as a constant. In another embodiment, a second switch controlled by an automatic gain control signal is established between a first stage amplifier and a second stage amplifier to improve dynamic range and bandwidth without degrading other amplifier specifications.

FIELD OF THE INVENTION

The invention relates to transimpedance amplifiers and in particular toa method and apparatus for increasing dynamic range, linearity, andstability of a transimpedance amplifier.

RELATED ART

FIG. 1 illustrates an example environment of use. In this exampleenvironment, an optical fiber 100 provides an optical signal to aphotodetector 104. The photodetector 104 converts the optical signal toa low power electrical current. To facilitate downstream processing theresulting electrical signal must be amplified. The output from thephotodetector 104 feeds into an amplifier 108, which increases the powerlevel of the input to a level suitable for downstream processing anddecoding. One type of amplifier often utilized for this application is atransimpedance amplifier (TIA), which is a low noise current to voltageamplifier.

The output of the amplifier 108 feeds into an equalizer 112. In thisconfiguration the equalizer 112 comprises an electronic dispersioncompensation (EDC) equalizer configured to correct for known signaldispersion: within the fiber. As is understood in the art, at certaintransmit rates, optical signals traversing fiber optics suffer fromknown dispersion, which distorts the transmitted signal. The EDCequalizer is configured with a transfer function inverse to that of theoptical fiber to thereby reverse the effects of the fiber.

For the equalizer 112 to accurately reverse the dispersive effects ofthe fiber channel, it is essential for the signal presented to theequalizer to be an accurate amplification of the signal from the fiber100. As a result, it is essential for the amplifier 108 to haveexcellent linearity, avoid clipping, and otherwise be free from unwanteddistortion. The equalizer is primarily configured to combat dispersionand as a result, distortion introduced by the amplifier 108 can not becorrected by the equalizer.

While prior art amplifiers performed acceptably at lower rates, such asrates in 1 to 2 gigabit per second transmit range, as transmit ratesincrease, prior art amplifiers suffered from increased non-linearity anddistortion. As a result, there is a need in the art for a linear andnon-distorting TIA amplifier for high rate applications.

SUMMARY

To overcome the drawbacks in the prior art, disclosed herein is anamplifier having an input configured to receive an input signal andhaving a degeneration resistance. The amplifier also includes anamplifier feedback loop configured to receive a control signal such thatthe control signal controls the feedback resistance which in turncontrols the gain of the amplifier. Also part of this embodiment is areplica circuit configured to replicate the amplifier and amplifierfeedback loop to thereby create a feedback resistance in the replicacircuit. A replica circuit feedback loop configured to alter thedegeneration resistance in relation to the feedback resistance in thereplica circuit.

The replica circuit may be a scaled version of the amplifier and thefeedback loop. In one embodiment, the control signal comprises anautomatic gain control signal. The amplifier may comprise atransimpedance amplifier. A current mirror may be part of the replicacircuit feedback loop. It is further contemplated that the amplifierfeedback loop may further comprise a FET switch configured to receivethe control signal and the replica circuit feedback loop furthercomprises a FET switch configured to alter the degeneration resistancein relation to the feedback resistance in the replica circuit.

Also disclosed herein is a method for maintaining linearity in anamplifier comprising receiving an input signal at an input of anamplifier and then amplifying the input signal with the amplifier. Inthis embodiment the amplifier further comprises a feedback resistanceand a degeneration resistance. This method then creates a feedbackresistance in a replica circuit such that the replica circuit is areplica of the amplifier and the feedback resistance. For example, inone embodiment in the replica circuit, the feedback resistance maycomprise a ratio or be related to the feedback resistance in theamplifier. This method creates a degeneration resistance control signalbased on the feedback resistance in the replica circuit and thenproviding the degeneration resistance control signal to a degenerationresistance control device in the amplifier to thereby control thedegeneration resistance in the amplifier.

In one embodiment, the ratio comprises 1 to 1. In one embodiment, thedegeneration resistance control device comprises a FET switch. It iscontemplated that the method may further comprise a current mirrorconfigured to create the degeneration resistance control signal. In oneembodiment, the feedback resistance comprises resistance from a FETswitch resistance and a feedback resistor. The amplifier may have thebandwidth to support data rates of 10 gigabit per second or greater.

Another transimpedance amplifier is disclosed wherein the TIA comprisesa variable gain amplifier configured to receive and amplify a signal,wherein the gain is set by a gain control signal which modifies afeedback resistance. A replica circuit is also provided and configuredto create a replica circuit feedback resistance which is a ratio of thefeedback resistance of the variable gain amplifier. An amplifieramplifies a resistance control signal such that the resistance controlsignal is based on the replica circuit feedback resistance. Also part ofthis embodiment is a resistance control device configured to change adegeneration resistance in the variable gain amplifier. The degenerationresistance is changed in relation to the feedback resistance in thevariable gain amplifier based on the resistance control signal.

The replica circuit replicates one or more aspects of the variable gainamplifier. In one embodiment, the resistance control device comprises aFET switch. In operation, changing the degeneration resistance inrelation to the feedback resistance in the variable gain amplifiercomprises maintaining the product of the degeneration resistance and thefeedback resistance in the variable gain amplifier as generallyconstant.

Also disclosed herein is a transimpedance amplifier comprising an inputconfigured to receive an input signal and an amplifier configured toamplify the input signal to create an amplified signal. The amplifieralso has an output configured to output the amplified signal. A feedbackloop having a feedback resistance is part of the system and it has afeedback resistance comprising a switch resistance, a feedback resistorin parallel with the switch resistance, and a divider element. Thedivider element is in parallel with the feedback resistance and is inseries with the switch resistance to create a combined switchresistance. The divider element reduces the effect of changes in theswitch resistance on the combined resistance.

In one embodiment the divider element comprises a resistance. The switchresistance may comprise the resistance of a FET switch and the switchresistance may change with an input voltage to the FET switch. It iscontemplated that the TIA amplifier may further comprise a detectioncircuit configured to provide a gain control signal to the FET switch.

Also disclosed is an amplifier having improved performancecharacteristics. In this configuration, a first stage amplifier has oneor more first stage inputs and one or more first stage outputs and asecond stage amplifier has one or more second stage inputs and one ormore second stage outputs. The one or more second stage inputs connectto the one or more first stage outputs. Also part of this configurationare one or more impedance matching devices at the output of one or moreof the first stage amplifier and second stage amplifier. The one or moreimpedance matching devices have impedance controlled by one or morecontrol signals. A detection system is also provided and configured tomonitor an input to the first stage amplifier input or an amplifiedsignal from the first stage amplifier or the second stage amplifier andgenerate, based on the monitoring, one or more control signals tocontrol the resistance of the one or more impedance matching devices.

In one embodiment the amplifier further comprises a feedback resistorbetween a second stage input and a second stage output. The impedancematching devices may comprise FET switches. It is contemplated that theone or more impedance matching devices may comprise a first impedancematching device between the first stage amplifier and a second stageamplifier and a second impedance matching device at the one or moresecond stage outputs. The one or more impedance matching devices adjustthe impedance between the first stage amplifier and the second stageamplifier.

Also disclosed is a two stage amplifier comprising a first stageamplifier and a second stage amplifier. In this configuration the outputof the first stage amplifier connects to the input of the second stageamplifier. A variable impedance matching device is located between thefirst stage amplifier and the second stage amplifier such that thevariable impedance matching device has an impedance controlled by acontrol signal received by the variable impedance matching device. Adetection circuit is also provided and is configured to detect a peakpower level or average power level from a signal output from the twostage amplifier and based on the detection, generate the control signal.

The variable impedance matching device may comprise a FET switch. In oneembodiment, the second stage amplifier has differential inputs and theFET switch is connected between the differential inputs and the controlsignal changes the resistance of the FET switch. The amplifier mayfurther comprise a second variable impedance matching device at theoutput of the second stage amplifier and the detection circuit furtheroutputs a second control signal to the second variable impedancematching device. In one configuration the amplifier further comprisesone or more feedback resistors connected between an input and an outputof the second stage amplifier.

Also disclosed is a method for variably amplifying a signal comprisingreceiving a signal and amplifying the signal in a first stage amplifierto create a first amplified signal. Then presenting the first amplifiedsignal to a second stage amplifier and controlling the impedance betweenthe first stage amplifier and the second stage amplifier based on acontrol signal. The control signal controls the gain of the amplifier.Then amplifying the first amplified signal with a second stage amplifierand outputting the second stage amplifier on an output of the secondstage amplifier.

In one embodiment controlling impedance comprising adjusting aresistance between differential signal paths between the first amplifierstage and the second amplifier stage. The control signal may comprise anautomatic gain control signal. This method may further comprisecontrolling the impedance of an output of the second stage amplifierbased on a second control signal. Likewise, the method may furthercomprise monitoring the output of the second stage and creating thecontrol signal and second control signal based on the monitoring. In oneembodiment, controlling the impedance comprises providing the controlsignal to a FET switch.

Other systems, methods, features and advantages of the invention will beor will become apparent to one with skill in the art upon examination ofthe following figures and detailed description. It is intended that allsuch additional systems, methods, features and advantages be includedwithin this description, be within the scope of the invention, and beprotected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The components in the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.In the figures, like reference numerals designate corresponding partsthroughout the different views.

FIG. 1 is a block diagram of a prior art optical receiver front endhaving an amplifier and an equalizer.

FIG. 2 illustrates a circuit level block diagram with the dividernetwork to address the drawbacks in the prior art.

FIG. 3 illustrates a generalized block diagram of a circuit of FIG. 2.

FIG. 4 illustrates a block diagram of an example embodiment of a systemhaving a replica circuit.

FIG. 5 illustrates a circuit diagram of an example implementation of atransimpedance amplifier with a replica circuit.

FIG. 6 illustrates an example embodiment of a prior art amplifierutilizing a prior art solution for increasing dynamic range.

FIG. 7 illustrates an amplifier having multiple automatic gain controlelements with modified resistive network.

FIG. 8 illustrates an example circuit level diagram of the generalizedblock diagram of FIG. 7.

DETAILED DESCRIPTION

The example environment of FIG. 1 is but one possible environment of useand is provided to aid in understanding. It is contemplated that theinnovations disclosed herein may be utilized in any environment whichwould benefit from an accurate and linear TIA amplifier. After readingthe following disclosure, one of ordinary skill in the art may determinenumerous different environments of use for the disclosed innovations.

FIG. 2 illustrates a circuit level block diagram of one innovation toaddress the drawback in the prior art. This circuit illustrates anexemplary TIA device 200. In general, amplifier element 204 may bebroadly described as the amplifier while gain control loop 208 serves asa voltage based automatic gain control. Also part of the embodiment ofFIG. 2 is a collector resistor capacitance RcCc and a degenerationresistance capacitance RdCd. These elements are described below in moredetail.

In this circuit, an input 212 connects to a transistor 216, which ispart of the amplifier element 204. The transistor 216 has atransconductance value defined by g_(m) and is configured to generate anamplified output at outputs V_(out) and V_(e).

In operation and in this example environment, the input to the TIA 200varies with the intensity of the received optical signal and as aresult, the input signal swings between low to high current magnitudes.To account for such input swing, a voltage controlled feedback loop 208provides the amplifier feedback. Part of this loop is a FET switch 220and a feedback resistor 224. Controlling the FET switch 220 is a controlvoltage V_(agc), which determines the amount of resistance in thefeedback loop 208. By adjusting the resistance in the loop, theamplification of the amplifier element 204 is controlled. Greateramplification is applied to smaller input signals, while amplificationis reduced for larger input signals.

A drawback to this configuration is that the FET switch 220 causesunwanted distortion, which if uncorrected can hinder circuitperformance. This distortion occurs as a result of the FET resistance(R_(sw)) changing in response to voltage changes across the switch.These changes in resistance R_(sw) also change the resistance in thefeedback loop, which affects the linearity of signal amplification.

To overcome this drawback, an innovation is disclosed to reduce theeffect of the switch resistance R_(sw) on the overall loop by addingdivider element 230. The divider element 230 can be any device thatestablishes a series resistance in the feedback loop. The dividerelement 230 may be a passive element, active element or a combination ofboth.

In one embodiment, the divider element 230 comprises one or more fixedresistances. Through voltage division, a portion of the voltage acrossthe feedback loop is expressed across the divider element 230 while aportion is expressed across the FET switch 220. As compared to prior artembodiments which lacked the benefit of element 230, the innovationreduces the voltage swing across the switch 220. As a result, changes inswitch resistance R_(sw) have a smaller delta affect on the totalresistance across the loop. This in turn reduces the voltage changeresulting from changes in R_(sw). Testing including harmonic analysisreveals this divider element 230 improves performance.

FIG. 3 illustrates a generalized block diagram of the circuit of FIG. 2.In this block diagram, a feedback resistance 304 is in parallel with anamplifier 300. A variable FET resistor switch 308 receives a controlinput that determines the resistance presented by the switch, which inturn controls the resistance of the feedback loop. A series resistor 312is also provided in series with the variable switch. The series resistor312 could be fixed or variable. Operation occurs as described above inconnection with FIG. 2. The addition of the series resistance 312reduces the dominance of the variable switch resistance. This in turnreduces the change in feedback resistance resulting from changes in theresistance of variable switch.

Returning now to FIG. 2, a discussion of the drawbacks and principles ofoperation of the prior art is provided to aid in understanding of theinnovation presented next. As discussed above, the prior art TIA designshad linearity limitations at higher bit rates and when presented with arange of currents, such as 10 micro amps to 1 milliamp. Theselimitations included introduction of harmonic distortion and clipping.However, applications demand good linearity for low currents to highercurrents.

During operation of the prior art TIA, the input current coming in isamplified to give an amplified output. In response to increased current,the voltage swing across feedback resistor becomes larger and this inturn causes distortion or excessive gain.

In response, the V_(agc) value may be increased to lower the feedbackresistance value by providing a shunt around the feedback resistor R_(f)224. The input to the FET switch 220 is related to the value of theinput current. This operation occurs dynamically such that for low inputcurrents, the input to the Vagc magnitude is likewise small. In responseto larger input currents it is desired to have smaller feedbackresistance. In this manner, the swing across the feedback path isadjusted in an effort to maintain linearity and avoid clipping anddistortion.

For the circuit shown in FIG. 2, the FET switch 220 has a resistanceR_(sw) defined as follows:

$R_{sw} = \frac{1}{\beta\left( {V_{agc} - {V_{{PNA} -}V_{{ds} -}V_{T}}} \right)}$

As can be seen from this equation, the resistance R_(sw) dependsinversely on the input voltage to the FET switch 220. Thus, for largerinput currents, the swing across the drain-source terminals V_(ds)affects the switch resistance, which in turn adversely affects thelinearity.

The transimpedance of the amplifier is defined by the followingequation, where A is the gain of the amplifier and R′_(f) is thecombined feedback of the FET switch 220 R_(sw) and the feedback resistorR_(f).

${TZ} = \frac{{AR}_{f}^{\prime}}{1 + A}$

In addition, the bandwidth of the TIA is defined as follows, whereC_(pd) is the capacitance of the degeneration element.

${BW} = \frac{A}{2\;\pi\; R_{f}^{\prime}C_{pd}}$

From these two equations it can be seen that both the transimpedance andthe bandwidth are related to R′_(f) and as such, changes in R_(sw)affects these terms. In summary, as the input signal becomes larger,V_(agc) changes, changing the R_(sw). The swing across the FET switchcauses non-constant and non-linear behavior. Consequently, it isdesirable to eliminate or minimize the voltage swing across the feedbackFET switch.

In addition, transitioning from a low current input to a high currentinput changes the AC characteristic of amplifier behavior. For example,peaking can cause distortion. Some of these unwanted characteristicsflow directly from the AC response of the switch. This is often referredto as the dynamic behavior of the TIA in response to different inputcurrents.

Differing input current levels change the poles and zeros of theamplifiers so caution must be exercised to avoid instability problems.For example, if a TIA is configured to have certain AC response for lowcurrent, then in response to increased current, the transimpedancechanges. When this occurs, the pole locations move closer in vicinityand this can cause instability due to phase modulations. Any RCcombination in the circuit creates a pole or a zero.

In general, it is preferred to avoid positive feedback or unwantedoscillations and in general, negative feedback establishes and promotesstability. In a two pole system, the feedback can be delayed and createuncertain gain levels thereby causing instability.

In the example embodiment of FIG. 2, there exist two primary poles, withthe first pole being the dominant pole. The following equations providea representation for the dominant pole.

$w_{p\; 1} = \frac{A}{2\;\pi\; R_{f}^{\prime}C_{PD}}$

And the second pole is defined as

$w_{p\; 2} = \frac{1}{2\;\pi\; R_{c}C_{ctotal}}$

Thus, it can be seen that the pole locations depend on R′_(f) and R_(c).And, as set forth above, as the input current changes, so does R′_(f),which causes the pole 1 location to move higher on the pole/zero Bodeplot. This can then create instability. Note that pole 2 is dependant onthe collector resistance and capacitance and as such, changes in inputcurrent will generally not change the pole 2 location.

To overcome the drawbacks of the prior art, it is possible to adjust thegain (A) of the system to maintain separation between the poles or tomaintain the pole location constant, and thus maintain stability.Adjusting the gain A may occur in relation to the input current. It iscontemplated that the gain may be increased or decreased in relation tothe input current to maintain stability. In one embodiment, a decreasein the gain of the amplifier will maintain the separation between thepoles. In one embodiment these changes are in the Y domain or gaindomain.

The gain of internal amp A is defined as:

$A = \frac{g_{m}R_{c}}{1 + {g_{m}R_{d}}}$where R_(c) is the collector resistance and gm is the transconductanceof the transistor which is determined by the transistor dimensions,design, and current. In this embodiment the transistor is a bipolardevice, but it is contemplated that in other embodiments the transistorcan comprise any process. R_(d) is the degeneration resistance, whichmay be assumed to be the emitter resistance of the internal amplifier.Because all these values with the exception of R_(d) are constants, bychanging R_(d), the gain can be changed. Thus, increasing R_(d) willdecrease the gain A.

Deriving the equations for the gain A and the first pole yields thefollowing relation:

$w_{p\; 1} = \frac{g_{m}R_{c}}{2\;\pi\; R_{f({1 +}}^{\prime}g_{m}R_{d)}C_{PD}}$and from this equation it can be seen that the pole location can bemaintained if in response to R′_(f) decreasing, the value for R_(d) issimultaneously increased. Conversely, increases in R′_(f) are counteredby decreases in R_(d).

With the assumption that the product of gm and R_(d) is far greater thanunity, further simplification yields:

$w_{p\; 1} = \frac{R_{c}}{2\;\pi\; R_{f}^{\prime}R_{d}C_{P}}$

Therefore, to maintain the pole location and stability, the productR′_(f)*R_(d) should be maintained constant. This is true because R_(c)does not change for a given setting even with input current changes. Theonly values that change are R′_(f) and R_(d). R′_(f) changes withR_(sw), which changes with input current. Therefore, by adjusting R_(d)it is possible to control the gain, which maintains system stability.

To implement this innovation, another element or circuit is established.In one embodiment, the product of R′_(f) and R_(d) is maintainedconstant to maintain system stability.

Turning now to the method and apparatus for maintaining R′_(f) relatedto R_(d), one example embodiment is disclosed utilizing a replicacircuit.

FIG. 4 illustrates on example embodiment of such an exemplary system.FIG. 4 is similar to FIG. 1, and only the aspects of FIG. 4 which differfrom FIG. 1 are discussed herein. In contrast to FIG. 1, FIG. 4 includesa replica circuit 404. The replica circuit is configured to maintain avalue R_(d) in relation to the value R′_(f), thereby maintaining theproduct of the two at or near a constant value. In turn and as discussedabove, stability is maintained.

It is contemplated that the replica circuit comprises any configurationof hardware or software and may include active devices, passive devices,or both. The term replica circuit indicates that one or more aspects ofthe TIA circuit 108 are duplicated to create a circuit that behaves andreacts to inputs and environmental conditions in a manner similar to theTIA itself.

FIG. 5 illustrates a circuit diagram of an example implementation of areplica circuit. This is but one possible example embodiment of areplica circuit and as such it is contemplated that one of ordinaryskill in the art may generate variations on this circuit that do notdepart from the claims that follow.

In this example circuit, the original TIA 504 is shown with R′_(f) andR_(d) shown for purposes of reference. Replicating this circuit is areplica circuit 508. Replica circuit 508 is generally similar to the TIAcircuit 504. The replica circuit may be larger, smaller, or the samesize as the TIA circuit 504. In one embodiment, the replica circuit 508is a scaled down version of the TIA 504. By replicating the TIA, processis matched for both circuits during creation. As a result, instead oftapping into a node of the TIA 504, which changes the behavior of theTIA, a replica 508 is created which behaves similarly over process,temperature and bias currents. The replica 508 generates generallyidentical or similar node voltage and current flows, or a scaled versionthereof, as in the TIA 504.

A constant voltage source 516 is applied to the replica circuit R_(f)and the resulting current is provided to an amplifier circuit 520, whichextracts the current. The resulting current is mirrored by currentmirror 512 to create a reference voltage. The current mirror may be a1:1 ratio or other ratio. This reference voltage is provided as an inputto an amplifier 530. This reference voltage is related to R_(f) of thereplica circuit. The other input to the amplifier 530 is a negativefeedback from a switch 534. In this embodiment the switch 534 comprisesa FET switch.

In operation, the TIA 504 reacts to changes in input current is ascommonly understood which in turn change the value of R′_(f) 540. Tomaintain a desired relationship between R′_(f) 540 and Rd 544 a replicacircuit 508 is provided. Due to the duplicate nature of the replicacircuit 508, the value of R′_(f) in the replica circuit is likewisechanging in relation to R′_(f) 540.

Across R′_(f) in the replica circuit a constant voltage is provided fromvoltage source 516. The resulting current is provided to the amplifiercircuit 520 to create I_(ref), which is in turn mirrored by currentmirror circuit 512 to create a reference voltage as shown. The referencevoltage is input to the amplifier 530 to generate an output thatcontrols switch 534. The switch 534 controls the resistance R_(d).Stated another way, a replica circuit is created to detect voltageacross R_(f) replica (which indicates R_(f) of the TIA) and thisdetected voltage is processed and used to control a switch which changesR_(d). Thus, the resistance R_(d) is controlled to inversely match thechange in R′_(f) 540 to thereby maintain the product of R′_(f) and R_(d)as generally constant.

Equations can be used to better illustrate these relationships. Thevalue V_(ref) can be expressed as follows:

$V_{ref} = {{I_{ref}R} = {\frac{\Delta\; v}{R_{f}}R}}$

Thus, I_(ref) is related to V_(ref) and V_(ref) is related to or basedon the changing value R_(f). Furthermore,

$\frac{V_{{ref},\max}}{V_{{ref},\min}} = {\frac{V_{d,\max}}{V_{d,\min}} = \frac{R_{f\mspace{14mu}\max}}{R_{f\mspace{14mu}\min}}}$which shows that V_(ref) is related to V_(d) and R_(f) and it ispossible to create a V_(ref) that is inversely proportional to R′_(f).In addition, it can be established that I_(d) through R_(d) is roughly aconstant. The amplifier 530 can be created to establish V_(d) andV_(ref) the same. As such, then

$R_{d} = {\frac{V_{ref}}{I_{d}} = \frac{\left( \frac{\Delta\; v}{R_{f}} \right)R}{I_{d}}}$and deriving this further yields,

${R_{d}R_{f}} = \frac{V_{k}R}{I_{d}}$where the right hand side of this equation is a constant. Hence, it isshown that R_(d) times R_(f) is constant as desired to maintainstability.

Also disclosed herein is a method and apparatus for increasing thebandwidth of an amplifier system, without degrading gain and dynamicrange. As is understood, it is preferred for an amplifier to have a highgain, wide or high bandwidth, large dynamic range and minimal peaking atlower gain. Prior art solutions were able to improve one of theseperformance aspects, at the expense of other aspects.

FIG. 6 illustrates on example embodiment of a prior art amplifierutilizing a prior art solution for increasing bandwidth. In this priorart embodiment, inputs 604 V_(inp) and V_(inm) connect to a first stageamplifier 608, the output of which connects to a second stage amplifier612. Feedback resistors 616A, 616B connect as shown to provide feedbackto the second stage amplifier 612. The outputs from the second stageamplifier 612 connect to resistors R 620A, 620B, and the opposing endsof the resistors 620A, 620B connect to the drain and source of an FETswitch 624. The gate of the FET switch 624 is controlled by an automaticgain control voltage, defined as V_(agc). The resulting voltage isprovided on outputs 640 as V_(outp) and V_(outm).

In operation, an input voltage on input 604 is amplified by the firststage amplifier 608 and the second stage amplifier 612. The controlvoltage Vagc controls the gain between the input 604 and the outputs640. When presented with higher frequency signals, the resistors 620 andFET switch 624 appear as an RC element which lowers the bandwidth due tothe capacitance of the switch 624 which appears as a low pass filter.

To overcome the drawbacks of the prior art, it is proposed to eliminatethe resistors 620 and add another switch to the system of FIG. 6. Thisnew example embodiment is shown in FIG. 7. The elements of FIG. 7 whichare generally similar to the elements of FIG. 6 are not discussed again.In this embodiment, the output of the second stage amplifier 612connects directly to the source and drain of the second FET switch 624and form the outputs 640. In addition, a first FET switch 704 isconnected across the output and input of the first stage amplifier 608and the second stage amplifier 612 as shown. In this embodiment, controlvoltages V_(agc1) and V_(agc2) control the FET switches respectively.

In operation, the control voltages V_(agc1), V_(agc2) are presented tothe gate terminals to create two independent terminals to control thedynamic range. As such, by increasing the value of V_(agc1), such as forexample near V_(dd), the FET switch 704, in this embodiment a P channelFET, results in no effect on the incoming signal and the level ofamplification. Presented with a high value of V_(agc1), the FET appearsas an open circuit.

As V_(agc1) is decreased, the FET switch turns on and reaches a linearregion resulting in its source to drain resistance decreasing. As thisoccurs, the switch 704 appears to have control over the input and outputimpedance of the amplifiers 608 and 612. This in turn reduces gain andas such, the V_(agc1) signal controls gain.

When analyzing dynamic range, the focus is on large signals. Typicallyit is desired to have a lower gain when presented with a large inputsignal to avoid saturation. By lowering the values of V_(agc1) andV_(agc2), the output impedances of the first and second stages arereduced. By having two V_(agc) controls, the effect is multiplicativeand it is possible to obtain a larger dynamic range.

The control voltages V_(agc1) and V_(agc2) are generated by a detectioncircuit 712. The detection circuit may comprise any combination ofhardware, software, or both configured to monitor and process theincoming signal, the outgoing signal, or both to generate the controlsignals V_(agc1) and V_(agc2). It is contemplated that peak detection ormagnitude monitoring may occur as part of the detection operation.

In one embodiment, the values for V_(outp) and V_(outm) are feedback tothe detection circuit 712. In such an embodiment, averaging or peakdetection occurs and the result is subtracted or compared to athreshold. If the result is higher than the threshold, then one or moreV_(agc) values are adjusted to maintain the output within a desiredrange.

FIG. 8 illustrates one example circuit level diagram of the generalizedblock diagram of FIG. 7. In this example embodiment, the FET 804comprises the first FET switch and the FET 808 comprises the second FETswitch. The first amplifier stage is shown as a differential amplifierby current mirror 812 while the second gain stage is shown as adifferential amplifier by the second current mirror 816. The resistors820 represent resistors 616 in FIG. 7. This is but one exampleembodiment of a circuit level implementation and as such, it iscontemplated that other circuit arrangements are possible based on thisdisclosure. For example, it is contemplated that these principles maybeimplemented in TIA, equalizers, limiting amplifiers, crosspointswitches, or any other application benefiting from one or more ofincreased bandwidth, gain, stability, and dynamic range and limitedpeaking, and variable gain amplifiers (VGAs).

The following table sets forth simulation results for the innovation setforth herein.

Prior Art Prior Art Approach Approach Prior Art Comparison (R = (R =Approach Current Table 100 ohms) 35 ohms) (R = 18 ohms) Approach Gain 9.64 dB  9.64 dB 9.64 dB  9.5 dB Bandwidth  2.1 GHz 11.78 GHz 12.7 GHz12.78 GHz Dynamic 18.53 dB 12.01 dB  9.2 dB 19.12 dB Rang Peaking at   9 db — —  2.76 dB Lower Gain worst case worst case.

As can be seen from this table, the bandwidth of the prior art approachis only 2.1 GHz, which does not approach the higher bandwidths, such as10 GHz. As a result, the prior art does not meet specification. In theprior art, the resistance value may be lowered, as shown when R=18 ohms,but this degrades the dynamic range. The current approach, shown in thefinal column and as disclosed herein achieves the desired bandwidtharound 12.78 GHz. In addition, the other factors are not degraded aswould normally occur in prior art approaches. For example, the gain ismaintained and the dynamic range actually increases to 19.12 db.Finally, peaking is also improved.

It is further contemplated that even though the current approachdisclosed herein is presented with a focus on a linear amplifier, it canbe extended to limiting applications as well to achieve the bandwidthand dynamic range by changing the values of one or more components. Forexample, the value of Rs (see FIG. 2) may be reduced or varied toperform in such an environment.

While various embodiments of the invention have been described, it willbe apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible that are within the scopeof this invention. In addition, the various features, elements, andembodiments described herein may be claimed or combined in anycombination or arrangement.

1. An amplifier comprising: an input configured to receive an inputsignal an amplifier configured to amplify the input signal to create anamplified signal, the amplifier having a degeneration resistance; anamplifier feedback loop configured to receive a control signal, whereinthe control signal controls the feedback resistance which in turncontrols the gain of the amplifier; a replica circuit configured toreplicate the amplifier and amplifier feedback loop to thereby create afeedback resistance in the replica circuit; and a replica circuitfeedback loop configured to alter the degeneration resistance inrelation to the feedback resistance in the replica circuit.
 2. Theamplifier of claim 1, wherein the replica circuit is a scaled version ofthe amplifier and the feedback loop.
 3. The amplifier of claim 1,wherein the control signal comprises an automatic gain control signal.4. The amplifier of claim 1, wherein the amplifier comprises atransimpedance amplifier.
 5. The amplifier of claim 1, furthercomprising a current mirror as part of the replica circuit feedbackloop.
 6. The amplifier of claim 1, wherein the amplifier feedback loopfurther comprises a FET switch configured to receive the control signaland the replica circuit feedback loop further comprises a FET switchconfigured to alter the degeneration resistance in relation to thefeedback resistance in the replica circuit.
 7. A method for maintaininglinearity in an amplifier comprising: receiving an input signal at aninput of an amplifier; amplifying the input signal with the amplifier,the amplifier further comprising a feedback resistance and adegeneration resistance; creating a feedback resistance in a replicacircuit, the replica circuit comprising a replica of the amplifier andthe feedback resistance in the replica circuit comprising a ratio of thefeedback resistance in the amplifier; creating a degeneration resistancecontrol signal based on the feedback resistance in the replica circuit;providing the degeneration resistance control signal to a degenerationresistance control device in the amplifier to thereby control thedegeneration resistance in the amplifier.
 8. The method of claim 7,wherein the ratio comprises 1 to
 1. 9. The method of claim 7, whereinthe degeneration resistance control device comprises a FET switch. 10.The method of claim 7, wherein the amplifier comprises a transimpedanceamplifier.
 11. The method of claim 7, further comprising a currentmirror configured to create the degeneration resistance control signal.12. The method of claim 7, wherein the feedback resistance comprisesresistance from a FET switch resistance and a feedback resistor.
 13. Themethod of claim 7, wherein the amplifier has a bandwidth for data at 10gigabit per second or greater.
 14. A transimpedance amplifiercomprising: a variable gain amplifier configured to receive and amplifya signal, wherein the gain is set by a gain control signal whichmodifies a feedback resistance; a replica circuit configured to create areplica circuit feedback resistance which is a ratio of the feedbackresistance of the variable gain amplifier; an amplifier configured toamplify a resistance control signal, wherein the resistance controlsignal is based on the replica circuit feedback resistance; and aresistance control device configured to change a degeneration resistancein the variable gain amplifier, wherein the degeneration resistance ischanged in relation to the feedback resistance in the variable gainamplifier based on the resistance control signal.
 15. The amplifier ofclaim 14, wherein the replica circuit replicates one or more aspects ofthe variable gain amplifier.
 16. The amplifier of claim 15, wherein theresistance control device comprises a FET switch.
 17. The amplifier ofclaim 15, wherein the changing the degeneration resistance in relationto the feedback resistance in the variable gain amplifier comprisesmaintain the product of the degeneration resistance and the feedbackresistance in the variable gain amplifier as generally constant.